Due to forces exerted on a workpiece, the worktable and workpiece supporting structures of such machines, distortions and misalignments occur. These result in inaccuracies in the dimensions and shape of the final machined article. Where the dimensional errors are of the same order as the accuracy to which the article is to be machined it is impossible reliably to machine the article. Distortions misalignments and other error producing effects which can arise and tend to be cumulative must therefore be reduced to a level which is such that the magnitude of the cummulative error in the final article is much less than the error which can be permitted therein.
One of the primary controlling parameters in a machining operation is the accuracy with which the workpiece position relative to a machining tool is known and it is an object of the present invention to provide a system by which the position o the worktable and the workpiece thereon can be accurately determined relative to a machining tool so that if the latter can be moved in a precise manner to engage the workpiece, material can be removed therefrom to a known depth. When applied to a machine tool incorporating features such as are described in our copending applications (our references C421.01/L and C422.01/L to C427.01/L) a highly accurate machine tool can be obtained.
The invention is of particular relevance to a cylindrical grinding machine.